1. Field Of The Invention
The present invention relates to optical measuring devices, and more particularly to such devices using optic fibers.
2. Prior Art
In a known optical measuring device for measuring the pressure prevailing in a container, a flexible membrane is inserted into the walls of the container. The membrane has an outside reflecting surface. A light-conducting optic fiber is arranged to illuminate the membrane reflecting surface. A light-emitting fiber is arranged with its input end such that reflected light from the reflecting surface enters the fiber. At the other end of the fiber there is a light detector which senses the strength of the light entering the fiber. The reflecting surface of the membrane and the end surfaces of the fibers are arranged so that the light from the light-conducting fiber reflected into the light-emitting fiber by the reflecting surface depends on the pressure in the container. By analysis of the light from the light-emitting fiber it is therefore possible to measure the pressure in the container.
A measuring device of the above type, however, suffers from a great number of errors, or possibilities of errors, which render the measuring device unsuitable for measurements in which moderate or high demands on the accuracy of measurement are placed. Examples of sources of error are:
Instability of the light source and its drive circuits;
Instability of the light receiver and its amplifier;
Varying damping of the light-conducting fibers because of so-called micro-bending, which means that when the fiber is bent light is conducted out of the fiber and provides an uncontrollable damping of the transmitted light;
Varying damping at fiber joints and instabilities in fiber connections at light transmitters and light receivers; and
Instabilities in the fixtures of the fibers.
It is obvious that sources of error such as that mentioned above may cause considerable disturbances in the measurement and great unreliability of the measured values, and should therefore be eliminated or compensated in some way when reliable measuring values are required. The errors are particularly prominent when measuring under varying temperature conditions and when long measuring times are required, without the possibility of calibrating.